A spectral computed tomography (CT) scanner includes a rotating portion rotatably supported by a stationary portion. The rotating portion supports an x-ray tube, which emits poly-energetic radiation (x-ray photons) that traverses an examination region and an object or subject therein, and a detector array with one or more rows of energy-resolving detectors that detect radiation traversing the examination region and generate electrical signals indicative of the detected radiation.
The electrical signals are amplified and processed by a pulse shaper to generate pulses having peak amplitudes indicative of the energy of the detected photons. A discriminator compares the amplitudes of the voltage pulses with two or more thresholds that are set in accordance with different energy levels and produces a signal, for a threshold, in response to the pulse amplitude rising above the threshold. For each threshold, a counter counts the produced signals, and an energy binner bins the counts into bins corresponding to different energy ranges. A reconstructor employs a spectral reconstruction algorithm to reconstruct the detected radiation based on the binned data.
A suitable energy-resolving photon counting detector includes a direct-conversion detector with a cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe or CZT), or another direct conversion material. A direct-conversion detector generally consists of a block of semiconductor material disposed between two electrodes, a cathode and an anode, to which a voltage is applied across. Radiation illuminates the cathode side, and the x-ray photons transfer energy to electrons, which create a number of electron/hole pairs, with the electrons drifting towards anode pixels of the anode side.
Such a detector may include a metallization that surrounds each pixel anode; the metallization has been referred to as a controlling or steering electrode. Generally, the steering electrode is held at a negative electrical potential, relative to the pixel anode, but not more negative than the cathode electrical potential. This results in an electric field that guides the drifting electrons to the pixel anode. The anode, in response to receiving electrons, produces an electrical signal indicative thereof, which is conveyed to an integrated circuit (IC).
The anode for each pixel is physically and electrically bonded to a complementary bonding pad of the IC, which includes processing electronics that route the signal off the detector, for example, to the reconstructor. After bonding an IC to direct conversion material, the anode-to-IC interconnect is tested. This can be achieved by irradiating the direct conversion material and measuring the output of the detector or applying a voltage across the direct conversion material and the IC pads. In the latter case, measuring a leakage current would indicate a good interconnect whereas measuring no current would indicate a bad interconnect.
With direct-conversion photon counting detectors with detector pixel pitches (pixel center to pixel center distances) of one millimeter (1.0 mm) or less and anode diameters in a range of fifty micron (50 μm) to one hundred microns (100 μm), bonding yields of the bonds between the pixel anodes and corresponding bonding pads of the IC, using stud-bump or low temperature solder, has been less than one hundred percent (100%) such as between sixty and eighty percent (60%-80%). Detectors with such bond yields generally are either reworked or discarded, which can increase overall per detector cost, and consumes time.
One potential approach to increase the bond yield in general is to use multiple interconnects for the same detector pixel anode/bond pad pair. Unfortunately, direct-conversion photon counting detectors with steering electrodes and with detector pixel anodes having diameters in a range of fifty microns (50 μm) to one hundred microns (100 μm) are not well-suited for multiple bonds with the same detector pixel anode due to space limitations. Therefore, there is an unresolved need for other approaches for increasing the bond yield.